We have demonstrated the reproducible generation of transforming viruses from cultured chicken fibroblasts infected with a non-transforming avian leukosis virus, tdB77(Appendix, I). Each of the isolates contains multiple viral genomes and each induces the expression two polyproteins in virus-transformed cells. The genomes of these viruses (SKVs) appear to be defective and each contains the putative oncogene, ski, which is unique among avian retroviruses. Homologs of the ski gene exist in chicken DNA suggested a recombinational origin of SKVs.